Washington (CNN) -- BP has been trying to shut down an internal safety watchdog agency it set up under congressional pressure four years ago, according to sources close to the office and a leading congressman.

The Ombudsman Program was set up after a 2005 explosion at a BP refinery in Texas that killed 15 workers and a massive oil spill in Alaska the following year. Its chief, former federal judge Stanley Sporkin, would not comment for this story -- but a source inside his office told CNN, "I'm surprised we're still here."

The Washington-based office was set up to hear BP workers' safety concerns after investigations into the Texas City refinery explosion raised questions about whether employees feared retaliation for speaking up. Since then, 112 employees have filed complaints, and 35 of them have dealt with "system integrity or safety issues" that the office says are extremely serious.

But sources close to the office say BP doesn't like having independent investigators pursuing those complaints. A union representative told CNN that some workers who complained have faced retaliation. Jeanne Pascal, a former lawyer for the Environmental Protection Agency, agreed.

"They've been demoted, they've been terminated, they've also been blackballed," Pascal said.

BP spokesman Steve Rinehart said the company has a "zero-tolerance" policy toward retaliation and said it is unaware of any unresolved cases that violate that policy.

"Concerns raised internally or with the Ombudsman's office in respect to our operations are fully investigated and appropriate actions are implemented," he said in a statement supplied to CNN. He added, "If Ms. Pascal or others believe there are cases that have not been resolved appropriately, they can be raised through the ombudsman's office.

BP has promised Rep. Bart Stupak, the chairman of a congressional subcommittee investigating the April sinking of the Deepwater Horizon drill rig, that it will keep the watchdog office in place for another year. But Stupak said the head of BP's American subsidiary, Lamar McKay, told him earlier this year that the ombudsman's office was slated for elimination.

McKay had become the head of BP America in January 2009, more than a year before the the Deepwater Horizon disaster killed 11 workers and uncorked the worst oil spill in U.S. history. Stupak, D-Michigan, and other lawmakers met with McKay in January and March of that year.

"One of the first things Mr. McKay said was, 'I'm going to replace the ombudsman. I'm going to shut her down,' " Stupak told CNN. "He wasn't even on the job for more than a few weeks, maybe a month or two, and he wanted to shut down the ombudsman. We encouraged him not to do so."

Stupak said lawmakers were "shocked" that McKay would bring up the topic so soon.

"The logic was, 'Well, we will make things better,' " he said. "Well, I'm not sure."

BP has said it can do a good job investigating complaints through an established internal system -- without the ombudsman's office. But Pascal, who spent 26 years prosecuting polluters for the EPA, has called BP a "serial environmental criminal." She said BP repeatedly violates environmental laws, scoffs at safety regulations and treats U.S. government safety and pollution control officials as a mere nuisance.

The company paid record federal fines and pleaded guilty to a felony in connection with the Texas City explosion. It also pleaded guilty in 2007 to one count of criminally negligent discharge of oil, a misdemeanor, in Alaska. In 2009, the Justice Department filed a civil complaint alleging the company had violated clean air and water laws in Alaska.

That record indicates that BP considers safety "a secondary or tertiary concern," with no indication that will change, Pascal said.

"From my perspective, BP for a long time has been a company that is interested in profits first and foremost," she said. "Safety, health and the environment are subjugated to profit-making, and I do not think that has changed."